The 5-HT/2A receptor is thought to be the main mediator of the psychotomimetic effects of hallucinogens in humans. In this component of the PPG, we will study the role of 5-HT/2A receptor in the behavioral actions of hallucinogens using genetically altered mice. Mice with a null mutation of the 5-HT/2A receptor gene are unresponsive to many of the behavioral and physiologic effects of serotonergic hallucinogens. To better define the roles of the 5-HT/2A receptor in specific behavioral actions of hallucinogens, we will perform additional experiments (head twitch response, forced exploration of a novel environment an drug discrimination) on the wild-type and 5-HT/2A knockout mice using a broader range of hallucinogenic and non-hallucinogenic agonists. In order to study specific aspects of the interactions of hallucinogens with the human 5-HT/2A receptor, we will create strains of "knock-in" mice expressing the human wild-type or four different mutated versions of the human 5-HT/2A receptor in place of the endogenous receptor. Designed and evaluated in the other components of this PPG, these mutations test specific aspects of the interactions of hallucinogens with the human 5- HT/2A receptor: (1) the effect of agonist positioning on intrinsic positioning on intrinsic activity; (2) the effect of receptor conformation and G-protein coupling; (3) the effect of destabilizing the inactive form of the receptor; and eventually, (4) the effect of reducing receptor desensitization and internalization on the development of tolerance to the effects of hallucinogens-using a new construct. We will create these knock-in mice and examine their behavioral response to hallucinogenic and non-hallucinogenic agonists. These knock-in mice will also be used in ex-vivo experiments in the other components of this PPG to characterize the in vivo effects of the mutations on agonist-mediated signal transduction and gene expression. The creation and characterization of these humanized 5-HT/2A knock-in mice will allow the three components of this PPG to achieve through the planned collaborations a better understanding of the molecular, cellular, and behavioral aspects of hallucinogen interactions with the human 5-HT/2A receptor that underlie their effects.